The popularity of optically-readable media, such as for example, compact discs (CD) and digital versatile discs (also known as digital video discs, or DVD), has grown rapidly since its introduction. When compared to other competitive storage media types, the accessibility of data, fidelity, low manufacturing cost, reduced size and other features have made optically-readable media, such as CDs and DVDs, an overwhelming choice for manufacturers and users alike. As a result, the variety of information stored on optically-readable media is great. Several types of content that are stored on optically-readable media, such as for example copyrighted content, have unique requirements, however, and some common problems exist with distribution of data on current CD and DVD formats.
One of the most well known problems with unlimited use optically-readable media is the piracy of content stored on the optically-readable medium. Piracy results when individuals take advantage of their ability to frequently copy information, such as music or movies. Piracy undermines the value of copyright protection. Schemes that would limit the usable life of optically-readable media would be of great benefit to copyright owners.
One scheme developed to protect copyrights is the development of a limited play optically-readable media. These are optically-readable media that become unreadable after a predefined period of time.
Limited play optically-readable media not only benefit the copyright owner but also end users. For example, movie rental stores would be able to issue limited play DVDs that would not have to be returned. This application would reduce DVD rental to a single transaction, as opposed to two separate transactions, thereby reducing expenses and providing opportunities for reduced rental fees.
Additional uses of limited play optically readable media include, for example, trial offerings of music or software. Benefits from such offerings would include market testing, and inducement of subsequent purchases.
Advances in materials science have produced promising materials for the further development of optically-readable media. Liquid crystals are an example of on such material. Liquid crystal materials are a class of materials whose optical properties have been studied at length.
Liquid crystal materials generally have several common characteristics. Among these characteristics are a rod-like molecular structure, strong dipoles and/or easily polarizable substituents.
A distinguishing characteristic of the liquid crystalline state is the tendency of the molecules to point along a common axis, called the director. This is in contrast to molecules in the liquid phase, which have no intrinsic order. In the solid state, molecules are highly ordered and have little translational freedom. The characteristic orientational order of the liquid crystal state is between the traditional solid and liquid phases and this is the origin of the term “mesogenic” state, used synonymously with liquid crystal state. Crystalline materials demonstrate long range periodic order in three dimensions. An isotropic liquid has no orientational order. Substances that are not as ordered as a solid, yet have some degree of alignment are properly called liquid crystals.
The liquid crystal state is a distinct phase of matter observed between the crystalline (solid) and isotropic (liquid) states. There are many types of liquid crystal states (phases) and their characterization depends on the amount of order in the material. Examples of liquid crystal phases are nematic, smectic, and cholesteric.
Liquid crystals are anisotropic materials, and the physical properties of a liquid crystal system varies with the average alignment. If the alignment is large, the material is very anisotropic. Similarly, if the alignment is small, the material is almost isotropic.
An example of a type of liquid crystal material is cholesteric liquid crystals (CLC). CLC's are called CLC's whether they are derived from cholesterols or not and take their name not from the type of material but from the fact that they have a long range twist about the director. CLC's are an intermediate state of matter between a crystal and a liquid. In CLC materials, the geometrically anisotropic molecules are arranged in layers with their long molecular axes parallel to one another in one plane and displaced incrementally in successive layers to give a helical type of stacking. Cholesteric liquid crystals are helical with the length of the helix comparable to the wavelength of light (for example, 350 nm to 750 nm). An important characteristic of the cholesteric mesophase is the pitch. The pitch is defined as the distance it takes for the director to rotate one full turn in the helix. A by-product of the helical structure of the chiral nematic phase, is its ability to selectively reflect light of wavelengths equal to the pitch length, so that a color will be reflected when the pitch is equal to the corresponding wavelength of light in the visible spectrum. Altering the pitch length results in an alteration of the wavelength of reflected light. If the angle at which the director changes is made larger the pitch length tightens. The wavelength of the reflected light can also be controlled by adjusting the chemical composition, since cholesterics can either include exclusively chiral molecules or nematic molecules with a chiral dopant dispersed throughout. The dopant concentration can be used to adjust the chirality and thus the pitch length.
The liquid crystals selectively reflect polarized light of wavelengths equal to the helix pitch length, so that a color will be reflected when the pitch is equal to the corresponding wavelength of light in the visible spectrum. The polarization reflected is related to the helicity of the CLC. Cholesteric liquid crystals are known to be sensitive to temperature and pressure. That is, as the temperature of the crystal increases so does the pitch of the helix and so does its color. Specifically, it is known that an increase in temperature corresponds to a longer wavelength (red light) reflected so that the observed color changes from red to blue.